I have the exact same problem. My bookmarks are organized into several folders of which have many bookmarks, but whenever I open one of those folders through the bookmarks bar, it doesn't show all my bookmarks in those folders and neither it lets me scroll through them.
The Browser even disregards the order in which I put them and is choosing by itself what bookmarks to show there that fit the screen.
My current version of Opera GX is LVL 1 (core: 67.0.3575.87) and this started happening around last week. I don't have the same problem with the standard version of Opera, both synchronized to the same account by the way.